The objectives of this study are to determine the safety, pharmacology and imunogenicity of ICM3 when administed as an intravenous infusion to subjects with moderate to severe psoriasis and to determine the effects of ICM2 administration compared to placebo on psoriatic disease activity, sensitization to a hapen antigen and ex vivo T cell mitogenic response to PHA. Because T cell interaction with antigen presenting cells have been documented to be of importance in the pathology associated with psoriasis, blocking the interaction with a monoclonal antibody (ICM3) may be efficacious.